To minimize the volume or bulk of inductors and capacitors used for temporary energy storage in power converters, it is preferable to operate the converters at high frequencies, thereby improving the ratio of power to volume. Such devices typically employ p-v-n power diodes for high voltage rectification. However, the frequency at which a converter can be operated is limited by the corresponding required switching speed of the p-v-n power diodes. The high voltage power diodes usually exhibit a significant reverse-recovery switching delay and power loss during each reverse-recovery, which increases with frequency. Power diodes also tend to inject significant electro-magnetic interference (EMI), developed during reverse-recovery switching, into the converter circuitry.
Conventional p-v-n power diodes use a lightly-doped base layer to block high voltages and therefore require carrier injection into this lightly-doped base to obtain a high ON-state conductance. Although such devices have a relatively low forward voltage drop, they are limited in switching speed by the lifetime of the injected carriers.
Ultra fast recovery (UFR) p-v-n diodes have been developed, which are more than an order of magnitude faster than the conventional p-v-n diodes. However, since switching losses due to diode reverse-recovery increase proportionally with the switching frequency in a power converter, the switching speed of a converter using UFR diodes is still limited to less than 100 KHz.
Diode reverse-recovery occurs when a diode is rapidly switched from a forward conducting to a reverse blocking state. To increase the voltage rating of diodes, a lightly-doped base region, which has very low conductance, is added at the junction. During forward conduction, minority charge carriers are injected into this base region from both N and P sides of the diode, enabling a relatively low ON-voltage to be achieved. Before the diode can block reverse voltage, these charge carriers must be removed. Removal of the minority charge carriers from a base region of the diode occurs over a reverse-recovery time. During this time interval, significant power loss is incurred due to simultaneous occurrence of a high reverse voltage and a reverse current. Reverse-recovery power loss increases with switching frequency and with the reverse-recovery time.
Currently, synchronous rectifiers comprising power metal oxide semiconductor field effect transistors (MOSFETs) are used in high voltage converters to achieve operation at frequencies above 100 KHz. The power MOSFETs in these devices are configured so that they turn on synchronously with other switches in the converter. Since MOSFETs are majority charge carrier devices with virtually no recovery time, they can be more efficient than p-v-n diodes. Their primary disadvantage arises due to the cost and complexity of providing an external control circuit for the gates of the MOSFETs so that they can be activated in synchronization with the other switches of the converter.
For power conversion at relatively low voltages (less than 100 volts), Schottky diodes are often used. Schottky diodes use a metal-semiconductor junction to provide current transport by majority charge carriers. Thus, like MOSFETs, Schottky diodes exhibit no observable reverse-recovery. However, to operate at higher voltages, a Schottky diode must be configured with a wide, lightly-doped base region, which limits the frequency response to about the level of a p-v-n diode, due to the relatively short lifetime of injected charge carriers.
A semiconductor device that operates as a diode and which starts conducting at a threshold voltage from anode to cathode of substantially zero volts is disclosed in published Japanese Patent Application No. H2-091974 (1990), which fists T. Sakai et al. as the inventors. This device, which is disclosed in three different embodiments, comprises an insulated gate type field effect transistor (FET) constructed with a channel formation region. This device has a second conductive semiconductor substrate deposited on a primary surface of a first conductive semiconductor substrate. A first drain region is provided inside a channel formation region, and in one embodiment, a groove is formed inside the drain region so as to extend into a first surface of the second semiconductor substrate; a gate-insulating film and gate electrode are then formed inside the groove. Art anode electrode electrically connects the channel formation region, the drain region, and the gate electrode. By proper selection of the impurity density in a P-type region underlying the gate insulating film and the thickness of the insulating film, a threshold voltage of the FET is set to about zero volts. If a voltage greater than the threshold is applied to the anode electrode (relative to the cathode that comprises a second surface of the first conductive semiconductor substrate), an N-channel is formed over the lower surface of the gate insulating film, and current flows through the device. This current comprises both a channel current and a body diode current. However, if the voltage on the anode is less than that of the cathode, channel and body diode current do not flow. Thus, the two terminal device achieves rectification at substantially a zero voltage threshold using a channel conduction effect. However, the device is useful only for relatively low current applications. Like most semiconductor devices, the current carrying capacity or conductance of the Sakai et al. FET is partly a function of its size; but for a given size, other more conventional and readily available devices can provide the same low forward voltage drop rectification at less cost.
Accordingly, it will be evident that a semiconductor diode device is required that exhibits minimal reverse-recovery, is operable at frequencies significantly above 100 KHz, and at a voltage substantially in excess of 100 volts, and which is capable of carrying substantial current compared to conventional rectifying devices of the same size. Such a device should have considerable, utility in power converters and in other applications in which high frequency, high power, and high voltage rectifiers are used.